There is currently a great interest world wide in providing inter-working between Telephony and Internet Protocol (IP) based networks in order to extend their respective services and advantages into the other network. One of the main reasons behind this interest resides on the higher flexibility and lower operating cost characterising the IP-based networks as transporting circuit switched network related signalling information between signalling nodes. An inter-working node acting as the border between corresponding domains, the Telephony domain and the IP domain, commonly represents such inter-working between Telephony and IP-based networks. This inter-working node is in charge of attending all the incoming requests from the IP domain as well as of sending all the traffic coming from the telephony domain to the IP domain.
One of the preferred protocols in the IP domain for call/session control is the Session Initiation Protocol (SIP), which is now under specification by the SIP Working Group of the Internet Engineering Task Forces (SIP IETF WG), within the Transport Area. In fact, several SIP entities, the so called Call Status Control Function (CSCF), have been defined in the third Generation Partnership Project (3GPP) which allow the Switched Circuit and the IP multimedia domains be interconnected.
In this respect, it is noticeable the effort in order to define protocol mapping mechanisms to make this inter-working possible between IP and Switched Circuit Networks. For example, the SIPPING Working Group within the Transport Area of the IETF defines the SIP-T framework to facilitate the interconnection of the Public Switched Telephone Network (PSTN) with the IP network. On the other hand, the Integrated Services Digital Network (ISDN) is nowadays a world wide spread network, shared by both fixed and mobile networks, wherein the ISDN User Part (ISUP) of a Signalling System #7 (SS7) is the signalling protocol that said ISDN makes use of. In this respect, the ISUP to SIP Mapping is another initiative from the SIP IETF WG, describing a way to perform the mapping between said two signalling protocols.
The current architectural proposals for this inter-working node, border between the Telephony and the IP domains, trend towards a Network Gateway (hereinafter referred to as NGW) approach, responsible for inter-working functions between Switched Circuit Networks running ISUP protocol and IP Packet Networks running SIP protocol for Call Control. Moreover, new signalling mechanisms are required in said NGW and other entities in the SIP domain to support Supplementary Services traditionally existing in the telephony domain like the aforementioned service for Call Completion on Busy Subscriber (hereinafter referred to as CCBS).
The SIP protocols or rather some of the emerging drafts in this area, like SIP Service Examples disclosed in the draft “draft-ietf-sip-service-examples-03.txt”, are describing how some existing ISDN/PSTN services may be extended into the SIP world. Nevertheless, there is no teaching or consideration yet in respect of how this typical telephony service, CCBS, may enter into operation as connecting Telephony and IP domains.